1. Field of the Invention
The present invention relates to a method for plugging lost circulation areas generated at the time when an oil or gas well is drilled with the use of a drilling mud and to a lost circulation material to be used therefor, which comprises a mixture of a hydrophilic urethane prepolymer and a water-soluble solvent containing no active hydrogen atoms and hardly miscible with mineral oils.
2. Description of the Related Art
In conventional way as shown in FIG. 3, in the drilling of oil wells, geothermal wells, etc., there has been used drilling muds 1 containing bentonite dispersed in water, so as to make it possible to carry away cuttings by a drill 2 and to form a clay wall. However, as shown in FIG. 4, the phenomenon 5 of lost circulation often occurs when a borehole is drilled through a porous formation, such as a sand or gravel layer. The lost circulation, if remains uncontrolled, not only causes the loss of drilling muds in large quantities, but also brings about such troubles as collapsing, caving and stucking. It is therefore of vital importance to clog up the areas of lost circulation as soon as it takes place.
Various methods have been employed to solve the problem of lost circulation. In one method, a drilling mud incorporated with a plugging material, such as shavings, rock wool, mica, granules of plastics, asbestos, glass fiber, etc. and a viscosifier, such as carboxymethyl cellulose, is circulated or supplied to the areas of lost circulation. However, this method suppresses lost circulation only temporarily, and in most cases lost circulation occurs once again soon after the resumption of the drilling. In one method employed to prevent the reoccurrence of lost circulation, water glass or cement is injected into the areas of lost circulation. However, the use of water glass is insufficient in its effect. The injection of cement is also insufficient in its effect and involves the danger of the stucking of the tubing. In addition, the hardening of both water glass and cement proceeds only slowly, and hence satisfactory results could hardly be attained.
An object of the present invention is to improve the method for plugging the areas of lost circulation occurring at the time when an oil or gas well is bored by the use of a drilling mud.
Another object of the present invention is to provide a lost circulation material to be used for the plugging of the areas of lost circulation.